In order to achieve high data rates, several technologies including Evolved Universal Mobile Telecommunications System (UMTS) terrestrial radio access (EUTRA) and EUTRA network (EUTRAN) have been developed in the Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) set of standards. Furthermore, local service requirements have led to the development of newer technologies in LTE-Advanced and are being proposed for “5G” implementations.
In order to provide local services, one approach is to use license exempt spectrum of wireless local area networks (WLANs). Another approach is data transmission on a licensed band in a coordinated and planned network. Regarding this latter approach, device-to-device (D2D) communication has been studied extensively, where wireless devices in proximity to one another can transmit data directly to each other with reused cellular resource blocks. Thus, in a D2D communication scenario, two wireless devices, such as user equipments (UEs), directly communicate with each other without having the payload traverse the backhaul network. Due to its local communication nature, D2D communication can be provided with smaller fees compared to the fees for cellular communication. D2D communication provides many benefits that cannot be provided by uncoordinated communication.
D2D communications provide a number of advantages. These are summarized as follows:                The overall network spectral efficiency can be improved significantly with an optimal configuration as compared with other communication technologies.        Low delay and low power consumption due to the proximity of wireless devices.        Improved radio resource utilization because of resource reuse by both cellular wireless devices and D2D pairs simultaneously.        Using one link for direct communication, instead of one uplink and one downlink for communication through the base station, reduces resource usage.        Offloading cellular traffic to D2D traffic reduces congestion in the backhaul network, benefiting existing cellular wireless devices in the network.        
There are many current and prospective applications for D2D communications. For example, D2D communications has been proposed for use in LTE-based public safety networks in the United States for its security and reliability. In addition, D2D communication is necessary for scenarios where the cellular transmission is not accessible.
In commercial networks, many social network applications require recognition of nearby wireless devices. Proximity wireless device recognition is usually handled in a centralized manner, where wireless devices are required to register their location information in a server such that the location information can be shared among the other wireless devices, e.g., in various social networking sites. With D2D, location registration is no longer required for the purpose of proximity discovery. Another prospective application for D2D communication is E-commerce, where nearby agencies need to transfer efficiently a large amount of private data.
One challenge of D2D communication is interference with coexisting cellular devices.
For a cellular network that includes D2D devices, interference needs to be carefully controlled because cellular devices and D2D devices share the spectrum. In order to manage the interference to the cellular devices in the same cell, several approaches have been proposed such as limiting D2D transmission power, for example, by employing a fixed booster factor and a back-off factor to adjust the D2D power.
In practical multi-cell networks, inter-cell interference (ICI) is a challenge that has not been addressed in the D2D literature. The ICI depends on the duplexing scheme used by cellular and D2D devices and the resources blocks shared between D2D and cellular devices.
Establishing direct communication between two nodes, or even among a set of nearby nodes in the Long Term Evolution (LTE) networks, is a way to enhance the spectral efficiency of the cellular network. Achieving potential improvements of D2D communication depends on efficiently addressing the resource and power allocation problems. Proposed solutions to these problems have proven inadequate. In a realistic cellular environment, there are multiple cellular devices and D2D pairs that attempt to access a shared resource pool. Normally, each node has access to multiple Resource Blocks (RBs) and also each RB is allocated to multiple interfering devices between cells (i.e., resource reuse among neighboring cells). Furthermore, either uplink or downlink resources can be used by D2D communication. Any realistic D2D resource allocation formulation should consider the aforementioned factors. No existing work has formulated all of these factors. Many studies propose simple heuristics that give highly sub-optimal performance, while the available optimal solutions are achieved under simplified cellular communication models.
D2D communication can cause significant ICI in neighboring cells. Current methods are not designed to maximize the combined sum rate of the cellular and D2D devices with a limit on the maximum ICI generated in the neighboring cell.
The D2D communication may be bi-directional communication where both devices receive and transmit in the same or different resources. However, D2D communication may also comprise scenarios in which one of the devices transmits and the other one receives the signals. There may also exist a point-to-multipoint (e.g. multicast, broadcast) scenario in which case a plurality of devices receive signals from the same transmitting device. This scenario is particularly useful for emergency services or public safety operation to spread vital information to several devices in an affected area. Of note, the terms D2D communication and D2D operation are interchangeably used herein.
Typically, wireless devices operate under the supervision of a radio access network with radio access nodes (e.g., base stations). However, in some scenarios the wireless devices themselves establish direct communication constituting the radio access network without the intervention of the network infrastructure.
In cellular network assisted device-to-device communications (or simply network assisted D2D communications), wireless devices in the vicinity of each other can establish a direct radio link, i.e., a D2D bearer. While wireless devices communicate over the D2D “direct” bearer, they also maintain a cellular connection with their respective serving base station (eNB). This direct link is interchangeably denoted as a network (NW) link, or a D2D-NW link. The NW link is used for resource assignment for D2D communication as well as maintenance of radio link quality of D2D communication link. As such, D2D communication is a promising feature that can potentially scale the capacity of the cellular networks.
Three relevant coverage scenarios for D2D communication have been defined as shown in FIG. 1. FIG. 1 is a block diagram of a wireless communication system 10 that includes a base station 12 that serves wireless devices in a region of coverage of the base station 12. The wireless devices may include two D2D wireless devices 14a and 14b, which may share physical resource blocks (“PRB s”). Throughout this disclosure, the term “D2D wireless device 14” shall refer to an individual D2D wireless device, i.e., either wireless device 14a or wireless device 14b. The term “D2D wireless device 14 pair” shall refer to both wireless device 14a and wireless device 14b. FIG. 1 also shows a cellular wireless device 16 in communication with the base station 12 and possibly also in communication with a D2D wireless device 14b. Note that herein a base station is but one example of base station 12. Implementations are not limited solely to base stations.
FIG. 2 is a block diagram of the wireless communication system 10 showing multiple base stations 12 each having a separate coverage area 18. In FIGS. 1 and 2, the solid connecting lines depict desired cellular or D2D transmissions, and the dotted connecting lines depict examples of interference from other cellular and D2D wireless devices.
In coverage: In this coverage scenario, all communicating D2D wireless device 14 pairs are within the network coverage. For example, the D2D wireless device 14 pairs can receive signals from and/or transmit signals to at least one network node such as the base station 12. In this case, the D2D wireless device 14 pairs can maintain a communication link with the network 10. The network 10 in turn can ensure that the D2D communication does not cause unnecessary interference. In coverage is also interchangeably referred to as in-network (IN) coverage.
Out of coverage: In this scenario, D2D wireless device 14 pairs communicating with each other are not under network node coverage. For example, the D2D wireless device 14 pairs cannot receive signals from and/or transmit signals to any of the network nodes. Typically the lack of coverage is due to complete absence of the network coverage in the vicinity of the D2D wireless device 14 pair. However, the lack of coverage may also be due to insufficient resources in the network nodes to serve or manage the D2D wireless device 14 pair. Therefore, in this scenario, the network cannot provide any assistance to the D2D wireless device 14 pairs. The out of coverage is also interchangeably referred to as out-of-network (OON) coverage.
Partial coverage: In this scenario, at least one communicating D2D wireless device 14 pair is within network coverage, and at least one other D2D wireless device 14 pair is not under network coverage, but is communicating with a D2D wireless device 14 that is under network coverage. As mentioned above, the D2D wireless device 14 not being under network coverage can be due to lack of any network node in its vicinity or due to insufficient resources in any of the network nodes in its vicinity. The partial coverage is also interchangeably called partial-network (PN).